Valve has nothing to do with it - Obsidian/Bethesda would have to spend money and development time to patch in workshop compatibility, and they likely don't consider it worth either since most people use the Nexus

There could be a lot of trouble with Nexus community when it comes to implementing top existing modes to Steam... Or they could just do it willingly and with pleasureHonestly, I just have torrent version of FNV to mod the sh&t of it while still playing vanilla on Steam.